Arnold Clas Robert Thunberg (5 April 1893 – 28 April 1973) was a Finnish speed skater who won five Olympic gold medals – three at the inaugural Winter Olympics held in Chamonix in 1924 (along with a silver and a bronze medal) and two at the 1928 Winter Olympics held in St. Moritz. He was the most successful athlete at both of these Winter Olympics, sharing the honour for 1928 Winter Olympics with Johan Grøttumsbraaten of Norway. No other athlete ever won such a high fraction of all Olympic events at a single Games. He was born and died in Helsinki.
Thunberg began with speed skating rather late, at the age of 18, having led a somewhat rowdy life as a compulsive smoker and drinker before he concentrated fully on his sport. However, from the age of 28 – when he turned up at his first European Allround Championships – and for the following ten years, he was by far the most-winning skater.
Thunberg's greatest strengths were the shortest distances, the 500 through 5000 metres. He never won an international 10000 metre event, although he did win a silver medal on the 10000 metres at the 1924 Winter Olympics – beaten by three seconds by compatriot Julius Skutnabb. Thunberg won three gold medals at the 1924 Olympics – the allround event, the 1500 metres and the 5000 metres. He remains the only person to have won an Olympic gold medal in allround speed skating, as despite the status of allround as the premier skating event, at least up until the 1990s, the event was abolished in the Olympics from 1928 onwards.